Fukushima Fish Still Radioactive

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More than a year and a half after an earthquake and tsunami
destroyed the Fukushima Dai-ichi nuclear power plant in Japan,
many fish in the area contain levels of radioactive cesium that
are just as high as they were soon after the disaster.

The finding suggests that the region's coastal-dwelling fish are
still being exposed to new sources of cesium, possibly from the
seafloor or from contaminated groundwater that's flowing into the
ocean. And even though most fish sampled in the new study had
levels of cesium below safe limits for consumption, some fish
contained surprisingly large amounts.

Japan has already closed fisheries near Fukushima to reduce human
exposure. The new results suggest that it may be a long time
before levels of radiation in the ocean decline after nuclear
disasters like the Fukushima meltdown.

"If (the cesium) is in the seafloor, it could be many years or
even decades for that to go away," said Ken Buesseler, an
oceanographer at the Woods Hole Oceanographic Institution in
Mass. "That implies we're going to have an issue in coastal
fisheries for a long time to come in Japan. We certainly can't
say we're out of the woods yet."

"Just because you haven't read about it in the news" lately, he
added, "doesn't mean it has gone away."

Because the Japanese rank among the most voracious consumers of
seafood in the world, the country's Ministry of Agriculture,
Forestry and Fisheries has been closely monitoring radiation
levels in coastal fish since the Fukushima disaster in March of
2011.

To play it extra safe, Japan has also tightened restrictions on
two radioactive forms of cesium so that fisheries must be closed
if levels in fish exceed a limit of 100 units of a measurement
called becquerels per kilogram of wet weight. Consuming levels of
cesium above that much every day for a year, Buesseler said,
would start to constitute a safety risk.

The fisheries agency has been releasing data regularly, including
an annual report on more than 8,500 samples of fish, shellfish
and seaweed taken from the coastal areas near Fukushima. Even
though the data is freely accessible to the public, it can be
hard to interpret so much information. So, Buesseler analyzed the
numbers to see what kind of patterns they might reveal.

He was immediately struck with the realization that radiation
levels in fish had not dropped in the past year. Fish naturally
lose a few percent of their concentrations of cesium if they are
not re-exposed, Buesseler said, suggesting that the animals are
still facing new inputs of the radioactive material.

Cesium levels were highest in fish that live near the seafloor,
Buesseler reported today in the journal Science. And
bottom-dwellers near Fukushima contained more cesium than did
fish that lived further away from the disaster site.

More than 40 percent of fish in the region contained levels
greater than the new safe-consumption limit of 100 units per kg.
Two greenling fish collected this August contained a surprisingly
high level of 25,000 units.

"The most intriguing thing in the study is that one would've
expected concentrations to decline significantly if cesium
concentrations in the water were declining over the last year,"
said Nicholas Fisher, an oceanographer at Stony Brook University
in New York, whose group detected small amounts of
Fukushima-derived cesium in bluefin tuna off the coast of
California last year.

"The fact that there's no significant decline in these fish
suggests that the fish are being exposed to a constant supply of
cesium either from their food or from the water."

Because bottom-dwellers contained more cesium than fish living
higher up in the water column, cesium may have accumulated in
sediments on the seafloor or in the worms and other invertebrates
that live in the sediments. And because it takes 30 years for
half of a sample of cesium to break down, Buesseler speculated
that decades could pass before exposure levels decline
significantly.

It's not clear exactly what the results mean for human health.
Fisheries near Fukushima remain closed, so it's not possible to
purchase seafood contaminated with cesium from the nuclear
meltdown.

Even if people were allowed to eat seafood from the area, the
Fukushima fish contain far lower levels of cesium than of
naturally occurring radioactive materials, including polonium-210
and postassium-40.

These materials are ubiquitous in marine animals, Fisher said,
because the Earth has always been radioactive and 99 percent of
radiation in the oceans is natural. Of the remaining one percent,
most comes from nuclear weapon testing in the 1960s.

"I don't give advice -- people can make up their own minds about
whether they'll eat seafood or not, but they should do so with
all available information," Fisher said. "They should know that
there's a lot of naturally occurring radioactivity."

For his part, Buesseler ate all of the seafood he was offered on
a trip to Japan in July.